Humans live a long time compared to other mammals of a similar mass. Not long enough my tastes, but this distinction of longevity has spurred a great deal of scientific investigation and theorizing over the years. In recent work, Joao Pedro de Magalhaes suggests an association with evolutionary optimization of mitochondrial function:
A gradual optimization of mitochondria - the cells' powerhouses - may have occurred in the human lineage, which could be associated with the evolution of human longevity and intelligence....
Humans are not only the smartest primates but have the longest lifespan, and hence these results could indicate a gradual optimization of mitochondrial proteins in the lineage leading to humans as a means to delay certain forms of neurodegeneration. "It has long been argued that longevity and intelligence evolved together in the lineage leading to humans," says de Magalhaes. "In fact, some nonhuman primates develop neurodegenerative changes at considerably earlier ages than what is typically observed in human patients. Mitochondria have been associated with neurodegenerative diseases, including the genes whose human disease-causing allele was found to be the normal allele in some nonhuman primates, and the mitochondrial genome has been linked to aging. So the general pattern of these results could indicate a selection on the human mitochondrion associated with the higher human intelligence and extended lifespan. Still," de Magalhaes warns, "we will need more detailed studies to prove this hypothesis."
The evolutionary value of delaying the onset of neurodegenerative conditions would be much the same as that proposed in the grandmother hypothesis - capable elders increase the survival rate of their descendants.